Modern wireless communications systems place great demands on the antennas used to transmit and receive signals, especially at cellular wireless base stations. Antennas are required to produce a carefully tailored radiation pattern with a defined beamwidth in azimuth, so that, for example, the wireless cellular coverage area has a controlled overlap with the coverage area of other antennas. The antennas may be deployed, for example, in a tri-cellular arrangement or, with a narrower beamwidth, as a six-sectored arrangement.
In addition to a defined azimuth beam, such antennas are also required to produce a precisely defined beam pattern in elevation; in fact the elevation beam is generally required to be narrower than the azimuth beam.
It is conventional to construct such antennas as an array of antenna elements to form the required beam patterns. Such arrays require a feed network in order to energise the antenna elements: on transmission, the feed network splits signals into components with whichever phase relationship is required to drive the antenna elements, and on reception, the feed network functions as a combiner. An array consisting of a single vertical column of antenna elements is commonly used at cellular radio base stations with a tri-cellular cell pattern. Similar arrays, but with two or more columns may be deployed if narrower azimuth beams are required. Generally, it is desirable to place antenna elements no more than approximately a half wavelength apart in azimuth at the carrier frequency under consideration to avoid generating grating lobes in the antenna pattern with associated unwanted nulls. It can be demanding to produce antenna elements physically small enough to be placed in an array on a half wavelength grid.
In addition the antenna should be capable of withstanding the environmental conditions experienced on the top of a mast, such as temperature extremes and wind loading, while being cheap to produce and light in weight to ease installation.
A design for an array antenna is described in the applicant's co-pending international patent application publication number WO 2007/031706; this design provides an antenna array having an electrically conductive tube (or cylinder), an electrically conductive outer surface, and a feed layer located between the tube and the outer surface. The antenna array is arranged to carry electrically conductive tracks, and houses dielectric material between the tube and the feed layer and between the outer surface and the feed layer. The antenna comprises a plurality of radiating elements formed as slots that are defined by areas of non-conductivity in both the front face of the outer surface and in the tube which are in registry with one another, the slots being energised in use by respective conductive tracks defined on the feed layer which are generally in registry with the slots.
In this design, the electrically conductive tube or cylinder—typically rectangular—may be made of a light weight plastics material with an electrically conductive coating with slots in the front face of the tube and ribs within the tube forming relatively closed cavities behind the slots. The tube therefore defines a relatively closed, compartmentalised but partially hollow structure. This presents some difficulty in manufacture because it is difficult to manufacture such structures as one-piece mouldings; as a result the antenna is likely to be moulded from two separate pieces, which are joined together to form the tube. This is a relatively expensive and time consuming.
It can be seen that there are many challenges to be faced when designing an antenna that produces a desired radiation pattern while being low cost and lightweight.